There is nothing wrong with what you propose.
In fact, if you can afford it, to cover the mortgage, it makes perfect sense. An empty house is asking for trouble. Copper is robbed daily throughout the country, at least you have a live in sitter. And i presume the tenant will let you in to do the place up as you wish.
Only word of warning is cover yer rear with a lease, and make sure you have a break clause in there. At some point u will want to raise rent, and your tenant may decide he wants to stay. Keep the lease short so you can ge out of the situation if required and dont let it fall into a part 4 tenancy by doing nothing.
If you dont know what im talking about you need to do some research on tenants rights.